Can you make this work:
Make it a 2-part pond, with the smaller pond uphill from the main, lower pond.
The upper pond would be where you could have certain plants, perhaps plants that prefer part shade (if this end of the system was under some trees) or additional filter media. Perhaps some form of media that would grow lots of bacteria. The upper 'pond' might be a large tank (like a water trough for animals). Or it could simply be another hole in the ground.
Then dig a winding stream bed for fairly slow water movement, lots of rounded stones, lots of emersed plants. At least 20' (7 meters) long, no more than a 2' drop along 20'.
The slow water movement will allow stuff to settle out of the water, and the emersed plants, growing in the stream will use that settling material to grow. Since they are in the air they have no lack of CO2, so can grow as fast as they want. The exposure of lots of water to the air will improve the gas exchange. Oxygen enters the water for the fish (if the plants are not producing much, like at night) and CO2 enters the water for the plants during the day.
Then a waterfall into the main pond. It does not need to be very high to be nice, and the splashing increases the gas exchange in the pond itself by causing ripples in the water.
The formula to make a nice sounding waterfall: You need 1000 gallons per hour for every 1' width of the waterfall. So if the waterfall is 18" wide, you need at least 1500 gph.
Metric: (very rough conversion) For every .3 meters width of waterfall you need 4000 liters per hour of water movement.
If you think you can move 56,000 gallons 2 times per day, that is not too far off 112,000 gallons per day. Divide by 24 = over 4,000 gallons per hour minimum. You could have a good waterfall with that.
I am not sure about sealing the pond, do you have access to Portland Cement? Does it mix with the local soil and set up? Around here, 5 bags of portland cement (100 lb bags) are mixed with 1 cubic yard (a bit less than 1 cubic meter) of a specific blend of sand and gravel. You might check with local builders who would know about products available to you.